Ptychognathus insolitus, Osawa, Masayuki & Ng, Ngankee, 2006

Osawa, Masayuki & Ng, Ngankee, 2006, A new species of Ptychognathus Stimpson, 1858 (Crustacea: Decapoda: Brachyura: Varunidae) from the Ryukyu Islands, southwestern Japan, Zootaxa 1260, pp. 57-66 : 58-65

Ptychognathus insolitus n. sp. ( Figs. 1–3 , 4 A–D)

Ptychognathus cf. hachijyoensis .— Kishino et al., 2001: 16, pl. 1, fig. 5. Not Ptychognathus hachijyoensis Sakai, 1955 .

Ptychognathus johannae .— Nakasone & Irei, 2003: 274 (key), fig. 50. Not Ptychognathus johannae Rathbun, 1914 a.

Type material. HOLOTYPE: CMNH­ZC 1960, male (cw 13.9 mm, cl 9.5 mm), Nakata, Izena Island, Ryukyu Islands, mouth of a narrow river, between pebbles, 23 May 2004, coll. M. Osawa. PARATYPES: CMNH­ZC 1961, 2 males (cw 9.8 mm, cl 6.9 mm; cw 9.9 mm, cl 7.1 mm), 2 females (cw 11.3 mm, cl 7.8 mm; cw 10.5 mm, cl 7.6 mm), 1 ovigerous female (cw 9.7 mm, cl 7.3 mm), same data as holotype.

Description. Carapace ( Fig. 1 , 2 A, B) transversely subrectangular in dorsal view, distinctly broader than long, greatest width across just behind anterior ends of cervical grooves, 1.3–1.4 times length; dorsal surface relatively flat, smooth, with scattered small pits; regions poorly defined; cervical grooves shallow but distinct; gastric grooves deep; epigastric cristae indistinct or weakly marked, separated by very shallow median sinus; postorbital cristae absent; no distinct oblique ridges on posterior branchial region. Anterolateral margin smooth, convex; external orbital angle very broad, terminating in blunt apex, outer margin arcuated; low epibranchial lobe present, separated from external orbital angle by shallow or indistinct cleft; anterior end of cervical groove indicated by indistinct or shallow cleft (no trace of indentation in holotype); postero­lateral margin gently convergent towards nearly straight posterior margin of carapace. Frontal margin ( Fig. 2 A, C) gently sinuous, somewhat deflexed, with low granules on median part and near internal orbital angle; shallow sinus present between 2 weakly convex lobes; narrow ridge present along frontal margin. Upper orbital margin gently sinuous, sub­parallel with frontal margin, with very low granules; lower orbital margin granulate, complete, confluent with anterolateral margin; distinct row of larger granules present just below lower orbital margin on suborbital region, extending to sub­branchial region.

Ocular peduncle ( Fig. 2 A) moderately well developed, slender, inflated proximally; cornea not dilated.

Basal segment of antennular peduncle ( Fig. 2 C) broad, roundly triangular in frontal view, with rugose ridge dorsally. Antennal peduncle ( Fig. 2 C) entering orbital hiatus, flagellum barely reaching distal margin of cornea of ocular peduncle.

Epistome ( Fig. 2 C) triangular, concave on outer surface; ventral margin with granular ridge, extending onto anterior part of buccal cavity as short ridge.

Third maxilliped ( Fig. 2 G) with ischium subequal in proximal and distal widths or some what increasing in width distally, lateral margin concave, ventral surface with longitudinal median ridge. Merus very broad, with antero­lateral angle strongly inflated, auriculiform; ventral surface with oblique ridge on mesial part. Exopod broad, inflated, greatest width 1.0–1.2 times of distal width of ischium.

Male chelipeds ( Figs. 1 , 3 A–C) equal or subequal, robust; surfaces nearly smooth, with scattered small pits. Merus with antero­ventral margin sinuous, rugose; dorsal surface with rounded, rugose crest; posterior surface slightly convex; posterior distal margin concave, slightly rugose; ventro­distal margin with strong, rounded projection. Carpus strongly inflated, unarmed. Palm broad, swollen on outer surface; dorsal surface rounded; ventral margin not delimited, rounded; inner surface with broad concavity on ventral part. Fixed finger terminating in hoof­shaped, corneous claw; outer surface with rounded longitudinal ridge ventrally; proximal part adjacent to cutting edge with dense tuft of long, plumose setae extending onto distal part of palm, setae reaching or overreaching distal tip of fixed finger; cutting edge with row of rounded or subacute teeth. Dactylus approximately as long as palm, gently curving, terminating in small, hoof­shaped, corneous claw; outer surface with dense tuft of long, plumose setae on proximal half; cutting edge with row of subtriangular, calcareous teeth decreasing in size distally. Moderately broad hiatus between fingers, base of fingers with narrow pulvinus (swollen area with thin cuticle).

Female chelipeds ( Fig. 3 D) distinctly smaller than those of male, covered with numerous, short stiff setae and several long setae; chela much narrower than that of male. Merus with row of small tubercles on antero­ventral margin. Carpus with small blunt spine at dorsodistal angle; dorsal surface with longitudinal, crenulate or protuberant ridge. Palm not strongly inflated, with scattered small, low tubercles; fixed finger with longitudinal ridge of small, low tubercles extending onto palm on outer surface ventrally. Dactylus with longitudinal ridge of small, low tubercles each on dorsal margin and outer surface ventrally. Narrow hiatus between fingers.

Ambulatory legs (second to fifth pereopods, Figs. 1 , 4 A–D) comparatively long slender, somewhat compressed laterally; fourth pereopod longest (length 1.5–1.7 times carapace width, 1.5 times in holotype); fifth pereopod shortest; propodus and dactylus of fifth pereopod much shorter than preceding legs; surfaces with scattered, small pits but nearly naked in male or with scattered, very short stiff setae in female; sparse, short and long, stiff setae present, ventral margins of propodi and dactyli most setose; all segments unarmed. Meri elongated, subrectangular in lateral view. Carpi each with shallow sinus along dorsal margin on lateral surface. Propodi slightly tapering distally in lateral view. Dactyli 1.2–1.4 (second pereopod), 1.2–1.3 (third), 1.3–1.4 (fourth), 1.0–1.1 (fifth) times as long as propodi measured along dorsal margin, each terminating in blunt or subacute corneous claw; dorsolateral and dorsomesial margins each delimited by crenulate or protuberant ridge; dorsal surface concave.

Anterior sternal plate ( Fig. 2 D) broadly triangular, with concave lateral margins. Abdominal cavity in male nearly reaching to suture between third and fourth sternites.

Male abdomen ( Fig. 2 E) narrow, subtriangular in general outline, nearly smooth, distinctly narrowed at base of telson; first somite not reaching bases of coxae of fifth pereopods (fourth ambulatory legs), with distinct transverse ridge; second somite short; lateral margins of second to fifth segments nearly straight or weakly convex, lateral margin of sixth somite strongly convex posteriorly; sixth somite longer than fifth; telson tongue­shaped, length 1.0–1.2 times basal breadth, 1.5 times longer than sixth somite; terminal margin broadly rounded. Female abdomen ( Fig. 2 F) broad; fourth somite broadest; telson broadly subtriangular, 0.4 length of basal breadth, 1.1–1.3 times longer than sixth somite measured on midline, terminal margin broadly rounded.

Penis of male situated at concave edge of eighth thoracic sternite. First gonopod ( Fig. 2 E–G) in male stout (stoutness more distinct in holotype than smaller male paratypes), nearly straight, reaching to anterior margin of fifth sternite; distal part covered with numerous stiff setae obscuring tip; corneous distal process short, rounded in lateral view, directed laterally. Second gonopod short, distal part cup­shaped. Vulva of female raised, convex, oval in shape.

Color in life. Body and pereopods pale yellow or pale brown, setae dark brown. Fingers of chelipeds white.

Habitat. The type specimens were found near the mouth of a narrow river, hidden among the interspaces of pebbles in the calm stream. Individuals occurred together with a species provisionally referred as Ptychognathus barbatus .

Kishino et al. (2001) recorded this species (as P. cf hachijyoensis ) from sand and pebbles in a freshwater spring at the depth of 0.2 m in Amami­oshima Island. Nakasone and Irei (2003: fig. 65, tab. 6) collected the species (as P. johannae ) at a site with a salinity of 30 ‰ in Benoki River of the northern part of Okinawa Island.

Distribution. So far known only from the Ryukyu Islands (Amami­oshima, Okinawa, and Izena Islands).

Etymology. The specific name is derived from the Latin insolitus (= unusual) in reference to the unusual shapes of the carapace and first male gonopods of the new species.

Remarks. Ng & Tomascik (1994) established the genus Orcovita for their new species, O. saltatrix Ng & Tomascik, 1994 . This genus now includes six species, all known as anchialine and/or cavernicolous crabs from the Philippines, Kakaban Island in Indonesia, Guam, Niue, and the Loyalty Islands (Ng et al. 1996, Ng & Ng 2002). Ng & Tomascik (1994) placed Orcovita close to Ptychognathus but markedly different from it in having proportionally broader and more rectangular carapace (width about 1.4 times length, whereas it is about 1.2 times or less in Ptychognathus ), supraorbital margin being parallel or subparallel to the frontal margin (distinctly sinuous or sloping posteriorly in Ptychognathus ), frontal median triangle (absent in Ptychognathus ), outer margin of the external orbital angle being proportionally much longer; relatively more elongated and slender chelipeds and ambulatory legs, and first male gonopods being proportionally shorter and stouter. The new species has the characters of Orcovita rather than those of Ptychognathus , except for the absence of a distinct frontal median triangle of the carapace. Nevertheless, this unusual species should still belong to Ptychognathus since it has a short dactylus of the fifth pereopod. Known species of Orcovita possess elongated, rather slender dactyli (see Ng et al. 1996: figs. 1, 2). Examination of specimens of P. hachijyoensis has revealed that this species also possesses relatively stout first male gonopods like those of Orcovita species. The stoutness of the gonopods is not a clear distinction between Ptychognathus and Orcovita at present. The systematics and taxonomy of Ptychognathus are now revised by the second author and will be discussed on details in separate publication.

Ptychognathus insolitus n. sp. is unique among the species of the genus because of the possession of the characters mentioned above. The new species somewhat resembles P.

hachijyoensis in possessing a carapace being almost smooth near the anterolateral margins and having one or two, very low lobes behind the external orbital angle, which are indicated by shallow or indistinct clefts, and the fingers of the cheliped each with a terminal tuft of stiff setae and those of male also each with a tuft of hairs at the base on the outer surface. However, P. hachijyoensis is immediately distinguished from P. insolitus by the narrower carapace (the carapace width is about 1.2 times the length vs. 1.3–1.4 times in P. insolitus ), stouter second to fourth pereopods (the length of the fourth pereopod is 1.2–1.3 times the carapace width vs. 1.5–1.7 times in P. insolitus ), and propodi and dactyli of those pereopods with dense tufts of plumose setae on the ventral margins (see Fig. 4 E, F).

The new species is identical with specimens recorded as P. johannae from Okinawa Island by Nakasone & Irei (2003) and provisionally assigned to P. hachijyoensis from Amami­Oshima Island by Kishino et al. (2001), although their material was not available for study. The figures provided by Kishino et al. (2001: pl. 1, fig. 5) and Nakasone & Irei (2003: fig. 50) clearly show the characters of the new species: very broad carapace with an indistinct notch on the anterolateral margin, and slender and long ambulatory legs with sparse, short and long stiff setae. Ptychognathus johannae differs from P. insolitus in having a narrower carapace with strongly produced external orbital angle and two, more clearly marked epibranchial lobes, and stouter ambulatory legs (see Rathbun 1914a: pl. 30, fig. 2).

Figures

FIGURE 1. Ptychognathus insolitus n. sp. Male paratype, cw 9.9 mm, cl 7.1 mm, CMNH­ZC 1960, dorsal view (left) and ventral view (right).

FIGURE 2. Ptychognathus insolitus n. sp. Male holotype, cw 13.9 mm, cl 9.5 mm, CMNH­ZC 1960 (A, C – E, G); male paratype, cw 9.9 mm, cl 7.1 mm, CMNH­ZC 1961 (B); female paratype, cw 11.3 mm, cl 7.8 mm, CMNH­ZC 1961 (F). A, carapace and ocular peduncles, anterior view (small pits and lateral setae omitted from left side); B, carapace, dorsal view, left; C, anterior part of carapace and left cephalic appendages, frontal view; D, anterior sternum, sixth abdominal somite, and telson, ventral view; E, F, abdomen and telson, posterior view in proximal three abdominal somites, ventral view in distal three abdominal segments and telson; G, left third maxilliped, ventral view. Scale bars: 1.0 mm.

FIGURE 3. Ptychognathus insolitus n. sp. Male holotype, cw 13.9 mm, cl 9.5 mm, CMNH­ZC 1960 (A – C, E – G); female paratype, cw 11.3 mm, cl 7.8 mm, CMNH­ZC 1961 (D). A, left cheliped, carpus and chela, dorsal view (plumose setae illustrated simplified); B, same, carpus and chela, outer view (plumose setae illustrated as simplified); C, same, fingers, posterior view; D, same, carpus and chela, outer view; E, left first gonopods, mesial view; F, same, lateral view; G, same, distal part, dorsal view. Scale bars: 1.0 mm.

FIGURE 4. Ptychognathus insolitus, n. sp., male holotype, cw 13.9 mm, cl 9.5 mm, CMNH­ZC 1960 (A – D); Ptychognathus hachijyoensis Sakai, 1955, male, cw 12.7 mm, cl 10.7 mm, CMNH­ ZC 1471 (E, F). A, left second pereopod, lateral view; B, left third pereopod, lateral view (scattered small pits omitted); C, left fourth pereopod, lateral view (scattered small pits omitted); D, left fifth pereopod, lateral view (scattered small pits omitted); E, carapace and ocular peduncles, dorsal view; F, left fourth pereopod, propodus and dactylus, lateral view (plumose setae illustrated as simplified). Scale bars: 1.0 mm.